Recently, optical communication transmission systems have been drastically changed. That is, these transmission systems have been changed from the IM-DD (Intensity Modulation-Direct Detection) system, which had been the system in mainstream use in the past, to a system in which coherent detection is conducted as typified by QPSK (Quadrature Phase Shift Keying). More particularly, research and development has been conducted on DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) systems in which signals are superimposed on an orthogonal polarization and a phase, as a target for optical transmissions of 40 Gbps or more. Some of these systems have been launched as products.
In the DP-QPSK systems, a coherent receiver is used as a key component. This coherent receiver includes a PLC (Planer Lightwave Circuit) type optical coherent mixer, a polarization beam splitter (PBS), a photo detector (PD), and a trans impedance amplifier (TIA). These components are shipped from a number of device vendors. For example, Patent Literature 1 discloses a polarization beam splitter using SiO2.
Further, as a future prospect, a small coherent receiver called “Generation 2” has been studied in the MSA (Multi Source Agreement). This small coherent receiver requires further miniaturization of the PLC. In particular, Si photonics that uses Si as a base material of an optical waveguide has attracted attention. In Si photonics technology, the refractive index difference between Si and SiO2 is large, and thus it is expected that a small bend radius due to strong light confinement will be achieved. In Si photonics technology, it is expected that PD integration using Ge, PBS integration using large form birefringence, and the like will also be achieved.